1. Field of the Invention
The present invention relates generally to coated optical lithography elements and methods of coating optical elements, and particularly to optical photolithography elements for use in below 240 nm optical photolithography systems utilizing vacuum ultraviolet light (VUV) lithography wavelengths no greater than about 193 nm, such as VUV projection lithography systems utilizing wavelengths in the 193 nm or 157 nm region.
2. Technical Background
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 240 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize ultraviolet wavelengths in the 193 nm region or 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have started to progress towards shorter wavelengths of light, such as from the 248 nm wavelength region towards the 193 nm region and may continue to progress to the 157 nm region. Commercial use and adoption of such shorter wavelengths in the manufacture of semiconductor integrated circuits has been hindered by the transmission nature of such vacuum ultraviolet wavelengths region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 240 nm such as 193 nm or 157 nm light has been also due to the lack of economically manufacturable optical coatings, optical elements with optical film coatings and optical film precursor materials. For the benefit of vacuum ultraviolet photolithography in these short wavelengths below 240 nm such as the emission spectrum VUV window of a F2 or an ArF excimer laser to be utilized in the manufacturing of integrated circuits there is a need for optical film coatings that have beneficial optical properties including good transmission and durability and that can be manufactured economically.
The use of oxide optical films such as Al2O3, SiO2 Y2O3, Sc2O3 at wavelengths below 248 nm is hindered by the low transmission nature of such oxide films at such short VUV wavelengths. At 193 nm such oxide optical films that may have tolerable transmission are troubled by the formation of high fluence laser radiation exposure defects such as color center formations when exposed to the powerful and damaging 193 nm optical lithography radiation. At 157 nm such oxide optical materials do not have sufficient transparency for use as thin film optical layers, which limits 157 nm optical films to fluoride crystal materials such as MgF2, AlF3, GdF3 and LaF3. Such fluoride crystal materials with below 240 nm transmission form thin film optical layers comprised of small crystallites separated by voids and often can have packing densities approaching as low as 0.80. Such fluoride crystallite film voids are troublesome contamination sites which are prone to attracting contaminating vapors such as water which can strongly absorb below 185 nm wavelengths. Attempts to increase crystalline sizes and minimize such voids such as by using energetic ion assisted deposition lead to absorbing film layers owing to the low energy of disassociation of such fluoride crystal materials. Options for increasing the packing densities of such fluoride crystal films such as by heating the optical surface substrate to about 300xc2x0 C. elevated temperatures is troublesome and particularly ill advised for high thermal expansion substrate materials such as CaF2.
The present invention overcomes problems in the prior art and provides a means for economically manufacturing high quality optical coatings that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.